In recent years, in the automotive field, to lower fuel consumption and cut the amount of emission of CO2, it has been demanded to make the car bodies lighter in weight while improving impact safety by making the car body members high in strength. To meet these demands, it is effective to use high strength steel sheet for the car bodies and parts. For this reason, demand for high strength steel sheet has been rising. To use high strength steel sheet for car bodies or parts etc., the high strength steel sheet has to be joined with other metal sheets, but there are the following such problems in joining them.
In the past, car bodies have been assembled and parts attached etc. mainly by spot welding. Spot welding has been employed even when joining a plurality of metal sheets including high strength steel sheets. In a joint formed by superposing a plurality of metal sheets and spot welding them in this way, the tensile strength is an important characteristic. As the tensile strength, there are a tensile shear strength (TSS) measured by applying a tensile load in the shear direction and a cross tensile strength (CTS) measured by applying a tensile load in the peeling direction.
A spot welded joint formed from a plurality of steel sheets having a 270 to 600 MPa tensile strength increases in CTS along with an increase in strength of the steel sheets. Therefore, in a spot welded joint formed by steel sheets having a 270 to 600 MPa tensile strength, problems relating to joint strength seldom occur. However, in a spot welded joint formed by a plurality of metal sheets including one or more steel sheets having a 780 MPa or more tensile strength, even if the steel sheets increase in tensile strength, the CTS does not increase or else decreases. This is because due to the drop in deformation ability, more stress concentrates at the weld zones, due to inclusion of large amounts of alloy elements, the weld zones are hardened, and due to segregation by solidification, the weld zones fall in toughness.
For this reason, in joining a plurality of metal sheets including one or more steel sheets having a 780 MPa or more tensile strength, art for improving the CTS has been sought. As one of the arts for solving this problem, there is the art of mechanical joining members without causing the matrix material to melt. Specifically, there is the art of stacking members to be joined such as a plurality of metal sheets, holding down the outer circumference of the punch by a blank holder preventing the metal sheets from springing up while driving in a rivet by the punch, and thereby mechanically joining the plurality of metal sheets with each other by the rivet.
However, in this art, there were the problem that since a rivet is driven in, the die side steel sheet deforms by an extremely great amount and, due to insufficient ductility or localization of deformation, the die side steel sheet fractures, the problem that when a tensile stress is applied in the shear direction and peel direction, the rivet will pull out and break and sufficient values of tensile strength in the shear direction and peel direction cannot be obtained, and the problem that there is almost no difference from the same rivet driving type of high strength steel sheet joints and mild steel sheet joints when comparing the fatigue strengths of the two.
As art for solving such problems, PLT 1 discloses the art of joining stacked high strength steel sheets with tensile strengths of 430 to 1000 MPa by driving a rivet through them and deforming the emerging front end of the rivet to thereby mechanically join the sheets and obtain a high strength steel sheet excellent in tensile properties and fatigue properties. The art disclosed in PLT 1 covers high strength steel sheet with a tensile strength of up to 619 MPa in its study and is effective as art when joining a plurality of steel sheets. However, in PLT 1, application of the above art to a plurality of steel sheets including high strength steel sheets with a tensile strength of 780 MPa or more was not studied.
Further, NPLT 1 describes that when joining high strength steel sheet and aluminum alloy sheet by driving in a rivet to mechanically join them, joining them without defect is possible up to a plurality of metal sheets including high strength steel sheet with a tensile strength of 590 MPa or so, but with a plurality of metal sheets including high strength steel sheet with a tensile strength of 980 MPa, the rivet cannot pierce through the high strength steel sheet.
In this way, in the art of driving a rivet into metal sheets to mechanically join them, usually a hole is not drilled into the members to be joined before joining them but the rivet itself is used to pierce through the members to be joined, so it was considered difficult to drive a rivet through a plurality of metal sheets including one or more steel sheets with a large deformation resistance, for example, steel sheets with a 780 MPa or more tensile strength, to mechanically join them.
As opposed to this, PLT 2 discloses a mechanical joining method joining thin-gauge sheets having high strength or work hardened to a high degree using a rivet wherein at the start of the joining process or right before it, a blank holder and die or components arranged next to the blank holder and die or components arranged in front of them are used to heat the thin gauge sheets restricted in location and time by electrical resistance heating.
In this way, PLT 2 describes art able to be applied to steel sheet having a high strength or work hardened to a high extent. It can be considered art effective to a certain extent even for a plurality of metal sheets including one or more high strength steel sheets with a tensile strength of 780 MPa or more. However, when using the art disclosed in PLT 2 to actually join together by a rivet a plurality of metal sheets including one or more high strength steel sheets with a tensile strength of 780 MPa or more, sometimes riveting is not possible. There was room for further improvement. Further, even with a metal sheet with a tensile strength of less than 780 MPa, if the processing speed when driving in the rivet becomes higher, the metal sheet becomes larger in deformation resistance and therefore similarly there was room for improvement.